Page 251 - Root Cause Failure Analysis

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PUMPS

Design, installation, and operation are the dominant factors that affect a pump’s mode
of failure. This chapter identifies common modes of failure for centrifugal and posi-
tive-displacement pumps.

CENTRIFUGAL

Centrifugal pumps are especially sensitive to variations in liquid condition (i.e., vis-
cosity, specific gravity, and temperature); suction variations, such as pressure and
availability of a continuous volume of fluid; and variations in demand. Table 19-1
lists common failure modes for centrifugal pumps and their causes.

Mechanical failure may occur for a number of reasons. Some failures are induced by
cavitation, hydraulic instability, or other system-related problems. Others are the
direct result of improper maintenance. Maintenance-related problems include
improper lubrication, misalignment, imbalance, seal leakage, and a variety of other
situations that periodically affect machine reliability.

Cavitation
Cavitation in a centrifugal pump, which has a significant, negative effect on perfor-
mance, is the most common failure mode. Cavitation not only degrades a pump’s per-
formance but also greatly accelerates the wear on its internal components.

Causes
Three causes of cavitation in centrifugal pumps are change of phase, entrained air or
gas, and turbulent flow.

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